Fiber optic sensors, especially fluorescence or photoluminescence-based sensors, have been used to determine analytes in solution at concentrations as low as picomolar in real time. In particular, fiber optic sensors have enabled the determination of metal ions, such as Zn(II), Cu(II), Cd(II), Co(II) and Ni(II), Fe(II), Mn(II), Pb(II), and Hg(II) in aqueous solutions such as fresh water, sea water, and cerebrospinal fluid. Fiber optic sensors to determine analytes may include a binding protein or other photoluminescent or calorimetric indicator system on the fiber optic sensor, which binds with the analyte and emits a signal that can be measured. Fiber optic sensors are well suited to determining metal ions in remote (e.g., at depth in the ocean) or inaccessible (e.g., inside the living brain) circumstances. The optical design of a fiber optic probe, however, necessarily introduces background that reduces signal-to-noise ratio. Moreover, the use of fiber optic sensors is expensive and unnecessarily complex in certain circumstances, for example in the case of finite samples that have been collected and stored in bottles.
An issue with respect to binding-type assays with trace analytes in finite samples is the potential for inaccurate measurement due to the amount of binding agent. For example, if one has 20 picomolar free zinc in a liter of water and a probe is introduced (fiber optic or otherwise) that contains one nanomole of a zinc binding protein or other reversible binding zinc indicator with a zinc binding affinity (Kd) of 4 picomolar, the binding protein may bind up essentially all of the free zinc because the concentrations of both the free zinc and the binding protein are above the 4 picomolar Kd for zinc binding to the protein. The problem arises because the fractional occupancy of the binding protein may be less than 10% because all of the free zinc available in the liter of water is bound and additional binding protein remains. If instead of a liter of water, the sample were a very large, essentially infinite bath, the fractional occupancy of the binding protein would be roughly 90%, and the measurement would be more accurate. Thus, for an accurate measurement in a finite bath, a small amount of binding protein is required to obtain a higher fractional occupancy. A fiber optic may be used to introduce a small amount of binding protein on a tip of the fiber optic, but has the disadvantages mentioned above. If binding protein is dispersed throughout the water sample, the concentration of fluorophores associated with the binding protein becomes very low, and any signal is difficult to acquire.